The invention described below is a control circuit cooperative with a pulsed neutron generator tube. Such a tube also requires certain power supplies.
Well logging techniques utilizing pulsed neutron irradiation are commercially attractive techniques. Through the pulsed neutron technique, the thermal neutron lifetime or the thermal neutron decay time of the strata of the earth formations in the near vicinity of a well are tested and analyzed. This enables compilation of elemental constituents and a variety of information can be obtained. As an example, the porosity of particular strata can be determined in the near vicinity of a borehole.
Typically, such devices comprise an evacuated tube enclosing a deuterium-tritium accelerator source. The neutron source is typically an evacuated and sealed glass, metal or ceramic housing comprising an outer envelope. Operative equipment within the envelope comprises a target which is electrically insulated and maintained at a relatively high potential. Another element is a source of ions to be accelerated toward the target by the high voltage maintained at the target. There is additionally a replenisher element to stablize the gas pressure within the evacuated envelope. Relatively low pressures are desirable for operation of the device.
The apparatus has a replenisher which incorporates a heater element surrounded by a surface capable of absorbing or emitting gas molecules into the evacuated tube envelope dependent on the temperature of the heater element. This enables the pressure within the chamber to be controlled. When the surface is heated, thermal emission of absorbed gases occurs. On cooling, the surface absorbs the gases from the atmosphere. The pressure of gas within the housing controls the supply of positively charged ions for acceleration toward the target, and, therefore, adjusts the rate of production of neutrons. This gas is ordinarily deuterium or deuterium mixed with tritium. Typically, the target is impregnated with tritium.
The electrostatic repulsive force between accelerated ions and the nuclei of tritium atoms at the target is overcome to enable nuclear fusion. This produces an unstable helium isotope (N=5) which decays by neutron emission of approximately 14 MEV energy level.
Operation of the tube during a logging run must be stabilized in a certain pattern. High output is generally desirable to promote an adequate irradiation. On the other hand, consistency is also desirable, at least in the form of a desired average current level to avoid source induced drift in the data. Moreover, the tube is pulsed periodically. The manner and technique of pulsing is, in part, set forth in U.S. Pat. No. 4,264,823 incorporated herein by reference.
Operation of a pulsed neutron generator tube thus requires cooperation of the supporting structure. One supporting structure is the power supply which forms the replenisher current. It is a fairly large current, typically ranging around two amperes. Accordingly, the oilwell logging tool must include a replenisher current supply capable of going as high as about five amperes maximum. The present invention includes an interlock system cooperative with the replenisher current supply to protect the pulsed neutron generator tube in the event of failure of the replenisher current power supply.
In similar fashion, the ion source must be pulsed with a relatively large voltage pulse, a typical value being in the range of about 2,000 volts. There is, therefore, incorporated in this disclosure supporting structure which is a high voltage pulsed power supply for the ion source. This disclosure sets forth a control circuit which interlocks with that source.
The target is impressed with a relatively high voltage. A high voltage power supply capable of perhaps 100,000 volts is normally required. In the event a malfunction deprives the target of its high voltage while replenisher current is applied to the neutron generator tube, the device may be damaged.
The target current is of interest. There is a range of current levels that is desirable for proper operation. If the target current is outside that range, it is indicative of damage. Excessive current may well damage the device. A current which is too low is indicative of other malfunctions.
A typical pulsed neutron generator tube is obtained from Kaman Sciences Corporation of Colorado Springs, Colo. The device is packaged within an evacuated chamber pressurized by SF6 (sulphur hexafluoride) gas. A loss of gas typically indicates an alarm condition. The system for driving the pulsed neutron generator tube is set forth in the previously referenced patent. This enables the tube to form neutron irradiation in a time dependent pattern to accomplish desired results. The system includes the tube enclosed within a fluid tight sonde suspended by a well logging cable in a borehole. The logging cable conducts signals of interest to the surface. Control and recording equipment are normally located at the surface.